Update systemd_enable.sh

Removed the explicitly-set `DISPLAY` variable as described in the following [issue](tmux-plugins#127)

Fixes being unable to copy to clipboard with `xclip` after suspending / unsuspending.

scripts/handle_tmux_automatic_start/systemd_enable.sh

@@ -23,7 +23,6 @@ template() {
 
 	[Service]
 	Type=forking
-	Environment=DISPLAY=:0
 	ExecStart=${tmux_path} ${systemd_tmux_server_start_cmd}
 
 	ExecStop=${resurrect_save_script_path}